1. Field of the Invention
This invention relates to an active vibration control system, and more particularly to an active vibration control system for suppressing vibrations or noise generated from prime movers or load devices driven thereby such as compressors and generators, or from apparatus equipped with engine exhaust mufflers or like intake and/or exhaust systems, or from running vehicles.
2. Prior Art
The term "vibration" used throughout the present specification includes not only vibration in its proper or literal meaning but also noise and sound.
Conventional active vibration control systems of this kind include a system which has been proposed by Japanese Provisional Patent Publication (Kohyo) No. 1-501344. The proposed system comprises, as shown in FIG. 1, a noise (vibration) source, an adaptive control circuit 102 which receives an output from the vibration sensor 101 as a reference signal and generates, based upon the reference signal, a cancelling signal having a transfer characteristic inverse to a transfer characteristic of vibration from the vibration source to a human body, a loud speaker 103 as cancelling vibration-generating means responsive to an output from the adaptive control circuit 102 for generating cancelling noise (cancelling vibration), and a microphone 104 as an error sensor for sensing a cancelling error between noise from the noise source and the cancelling noise from the loud speaker 103.
According to the above adaptive control circuit, noise (primary noise) picked up by the noise sensor 101 is sampled by an A/D converter 105, which supplies the resulting digital data as the reference signal X to the adaptive control circuit 102. The adaptive control circuit 102 in turn generates and supplies the cancelling signal to a D/A convertor 106 to be converted to a signal which drives the loud speaker 103 to generate cancelling noise (secondary noise).
On the other hand, the microphone 104 senses the cancelling error between the cancelling noise from the loud speaker 103 and the noise (primary noise) from the noise source, and the sensed cancelling error is sampled by an A/D convertor 107 into an error signal .epsilon. as digital data, which is fed back to the adaptive control circuit 102. Thus, the active vibration control system operates to vary the above-mentioned inverse transfer characteristic of the cancelling signal so as to minimize the value of the error signal indicative of the cancelling error between the primary noise and the secondary noise, to thereby suppress the noise from the noise source.
In the active vibration control system disclosed by Kohyo No. 1-501344, the adaptive control circuit 102 contains two FIR type adaptive digital filters (ADF) which selectively process only fundamental frequency components of the noise and higher harmonic components thereof.
The adaptive control circuit 102 also contains adaptive algorithm as a procedure for creating an optimal cancelling signal, which generally comprises LMS algorithm (Least Mean Square Method).
FIG. 2 shows another conventional active vibration control system which is a so-called multi-channel type active vibration control system capable of suppressing noise from a plurality of noise sources (vibration sources). This active vibration control system is comprised of noise sensors 108.sub.1 -108.sub.n, A/D converters 109.sub.1 -109.sub.n, D/A converters 110.sub.1 -110.sub.n, loud speakers 111.sub.1 -111.sub.n, microphones 112.sub.1 -112.sub.n, A/D converters 113.sub.1 -113.sub.n, n being equal to the number of the noise sources, and one adaptive control circuit 114 which operates to minimize the error between noise from the noise sources (primary noise) and cancelling noise (secondary noise).
The adaptive control circuit 114 contains a number n of control circuits provided respectively for the loud speakers 111.sub.1 -111.sub.n, which create cancelling signals for cancelling noise from the respective corresponding noise sources.
However, according to the above-mentioned conventional active vibration control systems, the frequency range of noise to be suppressed is limited to a low frequency range. In the system disclosed in Kohyo No. 1-501344 employing a plurality of adaptive digital filters for a single vibration source, only the fundamental frequency components and its higher harmonic components are selectively processed. That is, the conventional systems are not intended to suppress noise over its entire frequency range. Further, the adaptive digital filters used in these systems have such characteristics as to be able to suppress only low frequency noise components, making it impossible to process noise over a wide frequency range thereof.
For example, to suppress so-called random noise which has a wide frequency range, a system is required, which has the ability to suppress wide frequency range components. However, the conventional systems, which have low accuracy of cancelling noise components in a high frequency range, cannot satisfy such requirements, though they can suppress noise components in a low frequency range.
Moreover, component devices such as the noise sensors as vibration-sensing means, the error sensors, and the loud speakers as the cancelling vibration-generating means do not have uniform characteristics over the entire frequency range. However, in actuality, as each component device a single type is used, thus making it impossible to obtain satisfactory noise suppression effects over the entire frequency range.